You might call University of Pennsylvania researcher Michael Kahana a navigator of the mind. By pushing the limits of technology and research, he finds maps of the world in the brain that no one has seen before. His latest discovery, the human "grid cells" that help us navigate in space, required overcoming a particularly difficult challenge: Finding a way for study participants to activate the areas of the brain responsible for spatial navigation without leaving their hospital beds.
Ten years ago, Kahana was the first to discover human place cells, neurons that fire only when you're in a particular location. The "cognitive map" these cells generate becomes the stage where your brain replays spatial memories, imagines the way to get somewhere new, and, probably, animates dreamscapes when you sleep.
Now, Kahana's lab at the University of Pennsylvania, in conjunction with teams at Jefferson University Medical College in Philadelphia and UCLA, has discovered the first evidence of another navigational cell in the human brain previously found in monkeys, rats, and bats: Grid cells. Grid cells communicate with place cells by telling them about your location. Like your phone’s gyroscope, these grid cells determine location by analyzing movement information from your limbs, ears, and eyes. Unlike place cells, which only fire for one spot, each grid cell responds to a pattern of places, forming a triangular-lattice "grid" map of space.
Kahana says that these place-cell and grid-cell navigational systems work together to help us move through space.
"The cognitive mapping system probably relies on numerous representational variables," he says. "Place and grid cells are both important, and there are probably a bunch of other ones, too."
So grid cells aren't the only navigation cells in the brain, but they're a critical component of the system. Finding them in humans shows that the mammalian brain's version of GPS seems to span the evolutionary ladder, from rats to bats to people.
Kahana’s grid-cell study charted new territory, in this case a scientific frontier. Research ethics allow scientists to implant electrodes into animal brains, like mice, to record brain activity while they perform tasks like running through mazes. But implantation in humans, which requires invasive brain surgery, is completely unethical. So how did Kahana dive into the human brain?
He found study participants who already have electrodes in their head: Epileptic patients. People with epilepsy regularly have electrodes installed via brain surgery to monitor their seizures. Because seizures commonly originate from the temporal lobe, the part of cortex just under your temples that includes hippocampus and the amygdala, many epileptic recordings come directly from the parts of the brain involved in spatial navigation and memory. While having their brains clinically monitored in this way, many epileptic patients agree to let scientists gather data as well.
Even with willing participants with the necessary brain implants, Kahana ran into another problem. The 14 patients involved in the study had to be hooked up to wires and hospital equipment, so they couldn't very well be asked to walk a maze.
Instead, the scientists asked their subjects to navigate virtual space using a video game, which was developed by a comp-sci undergrad in Kahana's lab using PandaEPL, a library for programming spatial experiments created by Kahana's ex-student Alec Solway. The goal of the game is to bicycle around a cityscape, searching for four hidden objects.
Each patient searched for each object 12 times. As they learned the environment, they gradually got the hang of the routes, reducing their search time from 14 to 8 seconds on average. The virtual space they biked around was divided up into a 28-square array by the researchers. As the patients navigated from place to place, the scientists watched 893 cells in their entorhinal, hippocamal, parahippocampal, and cingulate cortices, as well as the amygdala, the mood-related structure buried deep in the temporal lobe.
Like all great explorers, Kahana understands that making discoveries like grid cells requires a combination of grit, determination, and the willingness to go where no one has before.
"You have to have the type of personality where when somebody says something can't be done, you say it can, just to be contrarian. That's the personality of a CEO, or somebody who starts their own business," says Kahana. "You have to be able to go deeper, or go to a new place where no one could go before, either because the technology didn't exist, or no one was willing to go to the effort. If everything already exists that you need to make the discovery, probably you were wrong. The way you get a real discovery is by literally breaking new ground."
By doing what no one had ever done before, watching human neurons fire during real-time navigation, Kahana found something new: Cells that fire at points on a spatial grid, just like the ones previously seen in animals. Maps in our heads.
Kahana wouldn’t let me run this article without mentioning one crucial fact about his research: There is no scientific evidence that women are worse at navigating than men, despite how many times reporters ask him the question.
"I always tell them the same thing: There's no scientific evidence to support the claim that women have worse spatial memory than men," says Kahana. "I've told [journalists] this every time; they've never quoted it. It's interesting to me that somehow society has decided women can't find their way around, and no scientific evidence in the world will convince them otherwise. Now let's talk about what you want to talk about. I'm done with my diatribe."
[Image: Flickr user Jason Tamez]